Solid golf ball

ABSTRACT

The present invention provides a solid golf ball having long flight distance obtained by accomplishing high launch angle and low spin amount, and having excellent shot feel, rebound characteristics, scuff resistance and yellowing resistance. The present invention relates to a solid golf ball comprising a core and a cover formed on the core, wherein  
     the cover comprises as a base resin  
     (a) polyurethane-based thermoplastic elastomer formed by using cycloaliphatic diisocyanate, or  
     (b) a mixture of polyurethane-based thermoplastic elastomer formed by using cycloaliphatic diisocyanate and polyamide-based thermoplastic elastomer, and has a thickness of not more than 1.0 mm, and  
     a surface hardness in Shore D hardness of the core is higher than a central point hardness in Shore D hardness of the core by not less than 20.

FIELD OF THE INVENTION

[0001] The present invention relates to a solid golf ball. Moreparticularly, it relates to a solid golf ball having long flightdistance obtained by accomplishing high launch angle and low spinamount, and having excellent shot feel, rebound characteristics, scuffresistance and yellowing resistance.

BACKGROUND OF THE INVENTION

[0002] Recently, ionomer resin has been widely used for cover materialof golf balls. This is because the ionomer resin is superior in reboundcharacteristics, durability, productivity and the like. However, sincethe ionomer resin has high rigidity and hardness, there are problemsthat in the resulting golf ball, shot feel is hard and poor; and spinperformance is not sufficiently obtained, which degradescontrollability, when using ionomer resin as the cover material.

[0003] In order to improve the problems, it is attempted to soften theionomer resin by various means. For example, it is proposed to blend thehard ionomer resin having high rigidity with terpolymer-based softionomer resin (Japanese Patent Kokai Publications No. 3931/1993,Japanese Patent No. 2709950 and the like) or blend the hard ionomerresin with thermoplastic elastomer (Japanese Patent Kokai PublicationsNos. 299052/1994, 327794/1994 and the like) to soften the cover.

[0004] However, when good shot feel and spin performance areaccomplished in case of the blend of the hard ionomer with the softionomer resin is used, rebound characteristics of the resulting golfball are largely degraded. In addition, since the surface of the coverof the golf ball is easily abraded by grooves on a face surface of thegolf club when hit by an iron club, it is problem that the surface ofthe golf ball becomes fluffy or begins to split finely, and scuffresistance is poor.

[0005] When the blend of the hard ionomer with the thermoplasticelastomer is used, rebound characteristics are excellent compared withthe blend with the soft ionomer resin, but the compatibility between theboth is poor, and scuff resistance when hit by an iron club is poorcompared with the blend with the soft ionomer resin.

[0006] In order to solve the problems, it has been suggested to usethermosetting polyurethane composition (Japanese Patent Kokaipublication Nos. 74726/1976, 241881/1992, U.S. Pat. No. 4,123,361 andthe like), or polyurethane-based thermoplastic elastomer (U.S. Pat. Nos.3,395,109, 4,248,432, Japanese Patent Kokai publication Nos.271538/1997, 1280401/1999, 1280402/1999 and the like) as a covermaterial of golf ball. When the thermosetting polyurethane compositionis used for the cover material, the scuff resistance, which is problemfrom using the blend of the hard ionomer resin with the soft ionomerresin or thermoplastic elastomer, is excellent. However, since a processof coating a cover on a core is complicated, large-scale production isdifficult, and the productivity is degraded.

[0007] When the polyurethane-based thermoplastic elastomer is used forthe cover material, compared with the thermosetting polyurethanecomposition, the productivity is improved, but shot feel,controllability and rebound characteristics of the resulting golf ballare not sufficiently obtained. When polyurethane-based thermoplasticelastomer formed by using typical diisocyanates, such as 2,4-tolylenediisocyanate, 2,6-toylene diisocyanate or a mixture thereof (TDI),4,4′-diphenylmethane diisocyanate (MDI) and the like, is used for thecover material, its strength is high, and scuff resistance of theresulting golf ball is excellent, but there is problem that yellowing iseasy to occur, and it is required to further use white paint. In orderto solve the problem, it has been suggested to use polyurethane-basedthermoplastic elastomer formed by using aliphatic diisocyanate for thecover material (Japanese Patent Kokai publication No. 271538/1997).However, there is problem that its strength is low, and scuff resistanceof the resulting golf ball is degraded, compared with thepolyurethane-based thermoplastic elastomer formed by using aromaticdiisocyanate.

OBJECTS OF THE INVENTION

[0008] A main object of the present invention is to provide a solid golfball having long flight distance obtained by accomplishing high launchangle and low spin amount, and having excellent shot feel, reboundcharacteristics, scuff resistance and yellowing resistance.

[0009] According to the present invention, the object described abovehas been accomplished by using as a base resin of the cover

[0010] (a) polyurethane-based thermoplastic elastomer formed by usingcycloaliphatic diisocyanate, or

[0011] (b) a mixture of polyurethane-based thermoplastic elastomerformed by using cycloaliphatic diisocyanate and polyamide-basedthermoplastic elastomer, and by adjusting the thickness of the cover andhardness difference between the surface and central point of the core tospecified ranges, thereby providing a solid golf ball having long flightdistance obtained by accomplishing high launch angle and low spinamount, and having excellent shot feel, rebound characteristics, scuffresistance and yellowing resistance.

[0012] This object as well as other objects and advantages of thepresent invention will become apparent to those skilled in the art fromthe following description with reference to the accompanying drawings.

BRIEF EXPLANATION OF DRAWINGS

[0013] The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

[0014]FIG. 1 is a schematic cross section illustrating one embodiment ofthe golf ball of the present invention.

[0015]FIG. 2 is a schematic cross section illustrating anotherembodiment of the golf ball of the present invention.

SUMMARY OF THE INVENTION

[0016] The present invention relates to a solid golf ball comprising acore and a cover formed on the core, wherein

[0017] the cover comprises as a base resin

[0018] (a) polyurethane-based thermoplastic elastomer formed by usingcycloaliphatic diisocyanate, or

[0019] (b) a mixture of polyurethane-based thermoplastic elastomerformed by using cycloaliphatic diisocyanate and polyamide-basedthermoplastic elastomer, and has a thickness of not more than 1.0 mm,and

[0020] a surface hardness in Shore D hardness of the core is higher thana central point hardness in Shore D hardness of the core by not lessthan 20.

[0021] In another embodiment, the present invention relates to a solidgolf ball comprising a core composed of a center and at least one layerof an intermediate layer formed on the center, and a cover formed on thecore, wherein

[0022] the cover comprises as a base resin

[0023] (a) polyurethane-based thermoplastic elastomer formed by usingcycloaliphatic diisocyanate, or

[0024] (b) a mixture of polyurethane-based thermoplastic elastomerformed by using cycloaliphatic diisocyanate and polyamide-basedthermoplastic elastomer, and has a thickness of not more than 1.0 mm,

[0025] a surface hardness in Shore D hardness of the center is higherthan a central point hardness in Shore D hardness of the center by notless than 15,

[0026] a surface hardness in Shore D hardness of the core is higher thanthe central point hardness of the center by not less than 20, and

[0027] the surface hardness of the core is higher than the surfacehardness of the center.

[0028] In order to put the present invention into a more suitablepractical application, it is desired that

[0029] the cycloaliphatic diisocyanate be at least one selected from thegroup consisting of 4,4′-dicyclohexylmethane diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane, isophorone diisocyanate, andtrans-1,4-cyclohexane diisocyanate; and

[0030] the cover have a Shore D hardness of 35 to 60 and a deformationamount when applying from an initial load of 98 N to a final load of1275 N of 2.50 to 3.20 mm.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The solid golf ball of the present invention will be explainedwith reference to the accompanying drawing in detail. FIG. 1 is aschematic cross section illustrating one embodiment of the solid golfball of the present invention. As shown in FIG. 1, the golf ball of thepresent invention comprises a core 1 and a cover 2 formed on the core.The core of the golf ball of the present invention is obtained bypress-molding a rubber composition under applied heat. The rubbercomposition comprises a base rubber, a co-crosslinking agent, an organicperoxide, a filler and the like. In the golf ball of the presentinvention, the core 1 may have a single-layered structure ormulti-layered structure, which has two or more layers.

[0032] The base rubber used in the present invention may be syntheticrubber, which has been conventionally used for cores of solid golfballs. Preferred is so-called high-cis polybutadiene rubber containing acis-1, 4 bond of not less than 40%, preferably not less than 80%. Thehigh-cis polybutadiene rubber may be optionally mixed with naturalrubber, polyisoprene rubber, styrene-butadiene rubber,ethylene-propylene-diene rubber (EPDM) and the like.

[0033] The co-crosslinking agent is not limited, but includes a metalsalt of α,β-unsaturated carboxylic acid, including mono or divalentmetal salts, such as zinc or magnesium salts of α,β-unsaturatedcarboxylic acids having 3 to 8 carbon atoms (e.g. acrylic acid,methacrylic acid, etc.), or a blend of the metal salt of α,β-unsaturatedcarboxylic acid and acrylic ester or methacrylic ester and the like. Thepreferred co-crosslinking agent is zinc acrylate, because it impartshigh rebound characteristics to the resulting golf ball. The amount ofthe co-crosslinking agent is preferably from 15 to 50 parts by weight,more preferably from 20 to 40 parts by weight, based on 100 parts byweight of the base rubber. When the amount of the co-crosslinking agentis larger than 50 parts by weight, the core is too hard, and the shotfeel of the resulting golf ball is poor. On the other hand, when theamount of the co-crosslinking agent is smaller than 10 parts by weight,the center is too soft, and it is required to increase the amount of theorganic peroxide in order to impart a desired hardness to the core.Therefore, the rebound characteristics are degraded, which reduces theflight distance.

[0034] The organic peroxide, which acts as a crosslinking agent orcuring agent, includes, for example, dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy) hexane, di-t-butyl peroxide and thelike. The preferred organic peroxide is dicumyl peroxide. The amount ofthe organic peroxide is preferably from 0.3 to 3.0 parts by weight, morepreferably from 0.4 to 2.0 parts by weight, based on 100 parts by weightof the base rubber. When the amount of the organic peroxide is smallerthan 0.3 parts by weight, the core is too soft, and the reboundcharacteristics of the resulting golf ball are degraded, which reducesthe flight distance. On the other hand, when the amount of the organicperoxide is larger than 3.0 parts by weight, the core is too hard, andit is required to decrease the amount of the co-crosslinking agent inorder to impart a desired hardness to the core. Therefore, the reboundcharacteristics are degraded, which reduces the flight distance.

[0035] The filler, which can be typically used for the core of solidgolf ball, includes, for example, inorganic filler (such as zinc oxide,barium sulfate, calcium carbonate and the like), high specific gravitymetal powder filler (such as tungsten powder, molybdenum powder and thelike), and the mixture thereof. The amount of the filler is from 2 to 50parts by weight, preferably from 3 to 35 parts by weight, based on 100parts by weight of the base rubber. When the amount of the filler issmaller than 2 parts by weight, it is difficult to adjust the specificgravity of the resulting golf ball, and a proper weight of the resultinggolf ball is not sufficiently obtained. On the other hand, when theamount of the filler is larger than 50 parts by weight, the weight ratioof the rubber component in the core is small, and the reboundcharacteristics are degraded.

[0036] The rubber composition for the core of the golf ball of thepresent invention can contain other components, which have beenconventionally used for preparing the core of solid golf balls, such asorganic sulfide compound, antioxidant, peptizing agent and the like. Ifused, the amount of the antioxidant is preferably 0.1 to 1.0 parts byweight, and the amount of the peptizing agent is preferably 0.1 to 5.0parts by weight, based on 100 parts by weight of the base rubber.

[0037] The core 1 is obtained by mixing the above rubber composition,and then vulcanizing and press-molding the mixture in a mold. Thevulcanization may be conducted, for example, by press molding in a moldat 130 to 180° C. and 2.9 to 11.8 MPa for 10 to 50 minutes.

[0038] It is desired that the core 1 of the golf ball of the presentinvention have a diameter of 40.8 to 42.2 mm, preferably 40.8 to 42.0mm, more preferably 41.0 to 41.8 mm. When the diameter of the core issmaller than 40.8 mm, the cover is thick, and the reboundcharacteristics are degraded. On the other hand, when the diameter islarger than 42.2 mm, the thickness of the cover is too thin, and thetechnical effects accomplished by the presence of the cover is notsufficiently obtained. In addition, it is difficult to mold it.

[0039] In the golf ball of the present invention, it is desired for thecore 1 to have a deformation amount when applying from an initial loadof 98 N to a final load of 1275 N of 2.50 to 3.20 mm, preferably 2.60 to3.10 mm, more preferably 2.70 to 3.00 mm. When the deformation amount issmaller than 2.50 mm, the core is too hard, and the shot feel of theresulting golf ball is hard and poor. On the other hand, when thedeformation amount is larger than 3.20 mm, the core is too soft, and theshot feel of the resulting golf ball is heavy and poor.

[0040] Generally, when the hardness difference between the surface andcentral point of the core is large, the launch angle is high and thespin amount is low as an important factor of flight performance, and theflight distance is improved. In the golf ball of the present invention,it is required for a surface hardness in Shore D hardness of the core 1be higher than a central point hardness in Shore D hardness of the core1 by not less than 20, and the hardness difference is within the rangeof preferably 20 to 40, more preferably 25 to 35. When the hardnessdifference is lower than 20, high launch angle and low spin amount arenot sufficiently accomplished, which reduces the flight distance. Inaddition, the impact force at the time of hitting is large, and soft andgood shot feel is not sufficiently obtained. When the hardnessdifference is too large, the durability is poor. Therefore, the upperlimit of the hardness difference is preferably 40, more preferably 35.

[0041] In the golf ball of the present invention, it is desired for thecore 1 to have a central point hardness in Shore D hardness of 30 to 50,preferably 32 to 48, more preferably 35 to 45. When the central pointhardness is lower than 30, the core is too soft, and the reboundcharacteristics of the resulting golf ball are degraded. On the otherhand, when the central point hardness is higher than 50, the core is toohard, and the shot feel of the resulting golf ball is poor. In addition,the launch angle is small and the spin amount is large, which reducesthe flight distance of the resulting golf ball. The term “a centralpoint hardness of the core 1” as used herein refers to the hardness,which is obtained by cutting the core into two equal parts and thenmeasuring a hardness at the central point in section.

[0042] In the golf ball of the present invention, it is desired for thecore 1 to have a surface hardness in Shore D hardness of 45 to 65,preferably 50 to 62, more preferably 55 to 60. When the hardness islower than 45, the core is too soft, and the rebound characteristics ofthe resulting golf ball are degraded. In addition, the launch angle issmall and the spin amount is large, which reduces the flight distance ofthe resulting golf ball. On the other hand, when the hardness is higherthan 65, the core is too hard, and the shot feel of the resulting golfball is poor. The term “a surface hardness of the core” as used hereinrefers to the hardness, which is determined by measuring a hardness atthe surface of the resulting core.

[0043] In the golf ball of the present invention, the core 1 may have asingle-layered structure or multi-layered structure, which has two ormore layers, as described above. When the core has a multi-layeredstructure, which has two or more layers, the solid golf ball of thepresent invention will be explained with reference to the accompanyingdrawing in detail. FIG. 2 is a schematic cross section illustratinganother embodiment of the solid golf ball of the present invention. Asshown in FIG. 2, the golf ball of the present invention comprises a core14 composed of a center 11 and at least one layer of an intermediatelayer 12 formed on the center, and a cover 13 formed on the core. Inorder to explain the golf ball of the present invention simply, a golfball having single-layer structured intermediate layer 12, i.e.three-piece solid golf ball, will be used hereinafter for explanation.However, the golf ball of the present invention may be applied for thegolf ball having two or more layers of core.

[0044] In the golf ball of the present invention, the center 11, whichis the inmost layer of the core 14, is preferably formed from the rubbercomposition as described above.

[0045] In the golf ball of the present invention, it is desired for thecenter 11 to have a diameter of 30.0 to 41.0 mm, preferably 31.0 to 40.2mm, more preferably 31.2 to 39.8 mm. When the diameter of the center 11is smaller than 30.0 mm, the intermediate layer is thick, and a desiredhardness distribution is not obtained. On the other hand, when thediameter is larger than 41.0 mm, the thickness of the intermediate layeror cover is too thin, and it is difficult to mold it.

[0046] In the golf ball of the present invention, it is required that asurface hardness in Shore D hardness of the center 11 be higher than acentral point hardness in Shore D hardness of the center by not lessthan 15, and the hardness difference is within the range of preferably15 to 25, more preferably 16 to 23. When the hardness difference islower than 15, the impact force at the time of hitting is large, andsoft and good shot feel is not sufficiently obtained. When the hardnessdifference is too large, the surface hardness of the center is harderthan the hardness of the intermediate layer, and a desired flightperformance is not obtained. Therefore, the upper limit of the hardnessdifference is preferably 25, more preferably 23.

[0047] In the golf ball of the present invention, it is desired for thecenter 11 to have a central point hardness in Shore D hardness of 30 to50, preferably 35 to 48, more preferably 38 to 45. When the centralpoint hardness is lower than 30, the center is too soft, and the reboundcharacteristics of the resulting golf ball are degraded. In addition,the shot feel is heavy and poor. On the other hand, when the centralpoint hardness is higher than 50, the center is too hard, and the shotfeel of the resulting golf ball is hard and poor. The term “a centralpoint hardness of the center 11” as used herein refers to the hardness,which is obtained by cutting the center into two equal parts and thenmeasuring a hardness at the central point in section.

[0048] In the golf ball of the present invention, it is desired for thecenter 11 to have a surface hardness in Shore D hardness of 45 to 60,preferably 48 to 58, more preferably 50 to 55. When the surface hardnessis lower than 45, the center is too soft, and the reboundcharacteristics of the resulting golf ball are degraded. On the otherhand, when the hardness is higher than 65, the center is too hard, thehardness difference from the intermediate layer is not sufficientlyobtained, and a desired launch angle and spin amount are not obtained.The term “a surface hardness of the center 11” as used herein refers tothe hardness, which is determined by measuring a hardness at the surfaceof the resulting center 11.

[0049] In the golf ball of the present invention, it is desired for thecenter to have a deformation amount when applying from an initial loadof 98 N to a final load of 1275 N of 2.80 to 4.00 mm, preferably 2.80 to3.80 mm, more preferably 2.90 to 3.50 mm. When the deformation amount issmaller than 2.80 mm, the center is too hard, and the shot feel of theresulting golf ball is hard and poor. On the other hand, when thedeformation amount is larger than 4.00 mm, the center is too soft, andthe shot feel of the resulting golf ball is heavy and poor. Theintermediate layer 12 is then formed on the center 11 to form the core14.

[0050] It is desired that the core 14 of the golf ball of the presentinvention have a diameter of 40.8 to 42.2 mm, preferably 41.0 to 42.0mm, more preferably 41.2 to 41.8 mm. When the diameter of the core 14 issmaller than 40.8 mm, the cover 13 is thick, and the reboundcharacteristics are degraded. In addition, the hardness difference inthe core is not sufficiently obtained. On the other hand, when thediameter of the core 14 is larger than 42.2 mm, the cover is too thin,and the technical effects accomplished by the presence of the cover isnot sufficiently obtained. In addition, it is difficult to mold it.

[0051] In the golf ball of the present invention, it is desired that asurface hardness in Shore D hardness of the core 14 (that is, a surfacehardness of the intermediate layer 12) be higher than a central pointhardness in Shore D hardness of the center 11 by not less than 20, andthe hardness difference is within the range of preferably 20 to 40, morepreferably 23 to 35. When the hardness difference is lower than 20, highlaunch angle and low spin amount are not sufficiently accomplished,which reduces the flight distance. When the hardness difference is toolarge, the core is cracked by deformation at the time of hitting, andthe durability is poor. Therefore, the upper limit of the hardnessdifference is preferably 40.

[0052] In the golf ball of the present invention, it is desired that thesurface hardness of the core 14 be higher than the surface hardness ofthe center 11, and the hardness difference is within the range ofpreferably 3 to 10, more preferably 4 to 9. When the surface hardness ofthe core 14 is not more than the surface hardness of the center 11, thehardness difference from the center is small, and a desired launch angleand spin amount are not obtained, which reduces the flight distance.

[0053] In the golf ball of the present invention, it is desired for thecore 14 to have a surface hardness in Shore D hardness of 55 to 75,preferably 58 to 70, more preferably 60 to 65. When the hardness islower than 55, a desired launch angle and spin amount are not obtained,which reduces the flight distance of the resulting golf ball. On theother hand, when the hardness is higher than 75, the durability of thecore is poor, and cracks occur. The term “a surface hardness of the core14” as used herein refers to the hardness, which is determined bymeasuring a hardness at the surface of the core 14 obtained by formingthe intermediate layer 12 on the center 11.

[0054] In the golf ball of the present invention, it is desired for thecore 14 to have a deformation amount when applying from an initial loadof 98 N to a final load of 1275 N of 2.70 to 3.50 mm, preferably 2.80 to3.40 mm, more preferably 2.90 to 3.30 mm. When the deformation amount issmaller than 2.70 mm, the core is too hard, and the shot feel of theresulting golf ball is hard and poor. On the other hand, when thedeformation amount is larger than 3.50 mm, the core is too soft, and theshot feel of the resulting golf ball is heavy and poor.

[0055] In the golf ball of the present invention, the materials used inthe intermediate layer 12 are not limited as long as the above hardnessdistribution is obtained, but may be the rubber composition as used inthe center 11, which comprises cis-1,4-polybutadiene as a base rubber,or thermoplastic resin, such as ionomer resin, thermoplastic elastomeror the mixture thereof. Preferred is the rubber composition or ionomerresin, because it imparts high rebound characteristics to the resultinggolf ball.

[0056] The ionomer resin, which can be typically used for the cover ofgolf balls, may be a copolymer of α-olefin and α,β-unsaturatedcarboxylic acid, of which a portion of carboxylic acid groups isneutralized with metal ion; a terpolymer of α-olefin, α,β-unsaturatedcarboxylic acid and α,β-unsaturated carboxylic acid ester, of which aportion of carboxylic acid groups is neutralized with metal ion; ormixtures thereof. Examples of the α-olefins in the ionomer preferablyinclude ethylene, propylene and the like. Examples of theα,β-unsaturated carboxylic acid in the ionomer include acrylic acid,methacrylic acid, fumaric acid, maleic acid, crotonic acid and the like,and preferred are acrylic acid and methacrylic acid. Examples of theα,β-unsaturated carboxylic acid ester in the ionomer include methylester, ethyl ester, propyl ester, n-butyl ester and isobutyl ester ofacrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acidand the like. Preferred are acrylic acid esters and methacrylic acidesters. The metal ion which neutralizes a portion of carboxylic acidgroups of the copolymer or terpolymer includes an alkali metal ion, suchas a sodium ion, a potassium ion, a lithium ion and the like; a divalentmetal ion, such as a zinc ion, a calcium ion, a magnesium ion and thelike; a trivalent metal ion, such as an aluminum ion, a neodymium ionand the like; and mixture thereof. Preferred are sodium ions, zinc ions,lithium ions and the like, in view of rebound characteristics,durability and the like.

[0057] The ionomer resin is not limited, but examples thereof will beshown by a trade name thereof. Examples of the ionomer resins, which arecommercially available from Mitsui Du Pont Polychemical Co., Ltd.include Hi-milan 1555, Hi-milan 1557, Hi-milan 1601, Hi-milan 1605,Hi-milan 1652, Hi-milan 1702, Hi-milan 1705, Hi-milan 1706, Hi-milan1707, Hi-milan 1855, Hi-milan 1856, Hi-milan AM7316 and the like.Examples of the ionomer resins, which are commercially available from DuPont Co., include Surlyn 9945, Surlyn 8945, Surlyn 6320, Surlyn 8320,Surlyn AD8511, Surlyn AD8512, Surlyn AD8542 and the like. Examples ofthe ionomer resins, which are commercially available from Exxon ChemicalCo., include Iotek 7010, Iotek 8000 and the like. These ionomer resinsmay be used alone or in combination.

[0058] As the materials suitably used -n the intermediate layer 12 ofthe present invention, the above ionomer resin may be used alone, butthe ionomer resin may be used in combination with at least one ofthermoplastic elastomer, diene-based block copolymer and the like.

[0059] Examples of the thermoplastic elastomers include polyamide-basedthermoplastic elastomer, which is commercially available from AtofinaJapan Co., Ltd. under the trade name of “Pebax” (such as “Pebax 2533”);polyester-based thermoplastic elastomer, which is commercially availablefrom Toray-Do Pont Co., Ltd. under the trade name of “Hytrel” (such as“Hytrel 3548”, “Hytrel 4047”); polyurethane-based elastomer, which iscommercially available from Takeda Badishes Urethanes Industries, Ltd.under the trade name of “Elastollan” (such as “Elastollan ET880”); andthe like.

[0060] The diene-based block copolymer is a block copolymer or partiallyhydrogenated block copolymer having double bond derived from conjugateddiene compound. The base bock copolymer is block copolymer composed ofblock polymer block A mainly comprising at least one aromatic vinylcompound and polymer block B mainly comprising at least one conjugateddiene compound. The partially hydrogenated block copolymer is obtainedby hydrogenating the block copolymer. Examples of the aromatic vinylcompounds comprising the block copolymer include styrene, α-methylstyrene, vinyl toluene, p-t-butyl styrene, 1,1-diphenyl styrene and thelike, or mixtures thereof. Preferred is styrene. Examples of theconjugated diene compounds include butadiene, isoprene, 1,3-pentadiene,2,3-dimethyl-1,3-butadiene and the like, or mixtures thereof. Preferredare butadiene, isoprene and combinations thereof. Examples of the dieneblock copolymers include an SBS (styrene-butadiene-styene) blockcopolymer having polybutadiene block with epoxy groups or SIS(styrene-isoprene-styrene) block copolymer having polyisoprene blockwith epoxy groups and the like. Examples of the diene block copolymerswhich is commercially available include the diene block copolymers,which are commercially available from Daicel Chemical Industries, Ltd.under the trade name of “Epofriend” (such as “Epofriend A1010”), thediene block copolymers, which are commercially available from KurarayCo., Ltd. under the trade name of “Septon” (such as “Septon HG-252”) andthe like.

[0061] The amount of the thermoplastic elastomer or diene blockcopolymer is 1 to 60 parts by weight, preferably 1 to 35 parts byweight, based on 100 parts by weight of the base resin for theintermediate layer. When the amount is smaller than 1 part by weight,the technical effect of improving the shot feel of the golf ball is notsufficiently obtained. On the other hand, when the amount is larger than60 parts by weight, the intermediate layer is too soft, and the reboundcharacteristics of the resulting golf ball are degraded. In addition,the compatibility with the ionomer resin is degraded, and the durabilityis degraded.

[0062] When the intermediate layer 12 is formed from rubber composition,the rubber composition for the intermediate layer is mixed, and coatedon the center 11 into a concentric sphere, and then vulcanized bypress-molding at 160 to 180° C. for 10 to 20 minutes in the mold toobtain a core 14, which is formed by covering the intermediate layer 12on the center 11. When the intermediate layer 12 is formed fromthermoplastic resin, the resin composition for the intermediate layer isdirectly injection molded on the center 11 to obtain the core 14. It ispreferable for the surface of the resulting core 14 to be buffed toimprove the adhesion to the cover formed on the core.

[0063] The cover 13 is then covered on the core 14 (or the cover 2 isthen covered on the core 1). In the golf ball of the present invention,it is required for the cover 2, 13 to comprise as a base resin

[0064] (a) polyurethane-based thermoplastic elastomer formed by usingcycloaliphatic diisocyanate, or

[0065] (b) a mixture of polyurethane-based thermoplastic elastomerformed by using cycloaliphatic diisocyanate and polyamide-basedthermoplastic elastomer. The amount of the component (a) or (b) is notless than 50% by weight, preferably not less than 70% by weight, morepreferably not less than 90% by weight, most preferably 100% by weight,based on the total weight of the cover.

[0066] Polyurethane-based thermoplastic elastomer generally containspolyurethane structure as hard segment and polyester or polyether assoft segment. The polyurethane structure generally contains diisocyanateand curing agent, such as amine-based curing agent. Thepolyurethane-based thermoplastic elastomer formed by usingcycloaliphatic diisocyanate is polyurethane-based thermoplasticelastomer that the diisocyanate is cycloaliphatic diisocyanate.

[0067] Examples of the cycloaliphatic diisocyanates include one orcombination of two or more selected from the group consisting of4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), which is hydrogenatedcompound of MDI; 1,3-bis (isocyanatomethyl) cyclohexane (H₆XDI), whichis hydrogenated compound of XDI; isophorone diisocyanate (IPDI); andtrans-1,4-cyclohexane diisocyanate (CHDI). Preferred is the H₁₂MDI inview of general-purpose properties and processability. Concrete examplesof the polyurethane-based thermoplastic elastomer formed by using theH12MDI include polyurethane-based thermoplastic elastomers, which arecommercially available from BASF Polyurethane Elastomers Co., Ltd. underthe trade name of “Elastollan XNY90A”, “Elastollan XNY97A”, “ElastollanXNY585”, and the like.

[0068] The diisocyanates generally include aliphatic diisocyanate andaromatic diisocyanate in addition to the cycloaliphatic diisocyanate. Asthe polyurethane-based thermoplastic elastomer, preferred arepolyurethane-based thermoplastic elastomers formed by using diisocyanatehaving no double bond in backbone structure in molecule, that is,aliphatic diisocyanate and cycloaliphatic diisocyanate in view ofyellowing resistance. Preferred are polyurethane-based thermoplasticelastomers formed by using cycloaliphatic diisocyanate and aromaticdiisocyanate, which have high mechanical strength, in view of scuffresistance. Therefore, in the cover used for the golf ball of thepresent invention, preferred is polyurethane-based thermoplasticelastomer formed by using cycloaliphatic diisocyanate in view of boththe yellowing resistance and scuff resistance.

[0069] Polyamide-based thermoplastic elastomer generally containspolyamide as hard segment and polyether or polyester as soft segment.The polyamide-based thermoplastic elastomer used for the cover 2, 13 ofthe golf ball of the present invention may be polyether-polyamide-basedthermoplastic elastomer or polyester-polyamide-based thermoplasticelastomer. Preferred is polyether-polyamide-based thermoplasticelastomer. Concrete examples of the polyether-polyamide-basedthermoplastic elastomer include polyamide-based thermoplastic elastomer,which is commercially available from Atofina Japan Co., Ltd. under thetrade name of “Pebax 5533SN00”, and the like.

[0070] The blend ratio (u:a) of the polyurethane-based thermoplasticelastomer (u) to the polyamide-based thermoplastic elastomer (a) in themixture is 100:0 to 70:30, preferably 98:2 to 80:20, more preferably95:5 to 90:10. When the amount of the polyamide-based thermoplasticelastomer is larger than 30% by weight, the scuff resistance of theresulting golf ball is degraded.

[0071] In the golf ball of the present invention, the base resin for thecover may include polystyrene-based thermoplastic elastomer,polyester-based thermoplastic elastomer, polyolefin-based thermoplasticelastomer, nylon resin and the like in addition to the abovethermoplastic elastomers.

[0072] The cover composition used in the present invention mayoptionally contain pigments (such as titanium dioxide, etc.) and theother additives such as a dispersant, an antioxidant, a UV absorber, aphotostabilizer and a fluorescent agent or a fluorescent brightener,etc., in addition to the above resin component as long as the additionof the additives does not deteriorate the desired performance of thegolf ball cover. If used, the amount of the pigment is preferably 0.1 to5 parts by weight, based on 100 parts by weight of the base resin forthe cover.

[0073] A method of covering on the core with the cover is notspecifically limited, but may be a conventional method. For example,there can be used a method comprising molding the cover composition intoa semi-spherical half-shell in advance, covering the core with the twohalf-shells, followed by press molding at 160 to 200° C. for 1 to 10minutes, or a method comprising injection molding the cover compositiondirectly on the core, which is covered with the cover, to cover it.Preferred is a method comprising press molding in order to obtain a golfball shaped into a true sphere.

[0074] In the golf ball of the present invention, it is required for thecover 2, 13 to have a thickness of not more than 1.0 mm, preferably 0.3to 1.0 mm, more preferably 0.5 to 0.9 mm. When the thickness is largerthan 1.0 mm, the cover is too thick, and the rebound characteristics ofthe resulting golf ball are degraded, which reduces the flight distance.When the thickness is too thin, the technical effects accomplished bythe presence of the cover are not sufficiently obtained, and it isdifficult to mold the cover such that the resulting golf ball is shapedinto a true sphere. Therefore the thickness of the cover is preferablynot less than 0.3 mm.

[0075] In golf ball of the present invention, it is desired for thecover 2, 13 to have a Shore D hardness of 35 to 60, preferably 40 to 50,more preferably 42 to 50. When the hardness is lower than 35, the coveris too soft, and the rebound characteristics are degraded. On the otherhand, when the hardness is higher than 60, the cover is too hard, andthe shot feel is hard and poor. The term “a hardness of the cover” asused herein refers to the hardness measured using a sample of a heat andpress molded sheets from the cover composition.

[0076] At the time of molding the cover, many depressions called“dimples” may be optionally formed on the surface of the golf ball.Furthermore, paint finishing or marking with a stamp may be optionallyprovided after the cover molded for commercial purposes.

[0077] In the golf ball of the present invention, it is desired to havea deformation amount when applying from an initial load of 98 N to afinal load of 1275 N of 2.50 to 3.20 mm, preferably 2.60 to 3.10 mm,more preferably 2.70 to 3.00 mm. When the deformation amount is smallerthan 2.50 mm, the golf ball is too hard, and the shot feel is hard andpoor. On the other hand, when the deformation amount is larger than 3.20mm, the golf ball is too soft, and the shot feel is heavy and poor.

[0078] The golf ball of the present invention is formed, so that it hasa diameter of not less than 42.67 mm (preferably 42.67 to 43 mm) and aweight of not more than 45.93 g, in accordance with the regulations forgolf balls.

EXAMPLES

[0079] The following Examples and Comparative Examples furtherillustrate the present invention in detail but are not to be construedto limit the scope of the present invention.

Production of Core (A) Single-layer Structured Core

[0080] The rubber compositions for the core having the formulationsshown in Table 1 were mixed, and then vulcanized by press-molding in themold at the vulcanization condition shown in the same Table to obtainspherical cores. The diameter, deformation amount, central pointhardness (1 a) and surface hardness (1 b) of the resulting core weremeasured, and the hardness difference (1 b-1 a) was determined bycalculation. The results are shown in the same Table. TABLE 1 Corecomposition I II III IV V BR-18 *1 100 100 100 100 100 Zinc acrylate 3333 33 33 33 Zinc oxide 12.0 12.5 12.0 12.0 11.5 Diphenyl disulfide *20.5 0.5 0.5 0.5 0.5 Dicumyl peroxide *3 1.0 1.0 1.0 1.0 1.0Vulcanization condition The first stage Temp. (° C.) 170 170 140 150 170Time (min) 15 15 20 30 15 The second stage Temp. (° C.) — — 165 — — Time(min) — — 8 — — Diameter of core (mm) 41.2 41.8 41.2 41.2 40.0Deformation amount of core 2.95 3.00 2.70 3.25 2.85 (mm) Core hardness(Shore D) Central point hardness (1a) 39 36 52 44 39 Surface hardness(1b) 59 60 55 54 57 Hardness difference (1b − 1a) 20 24 3 10 18

(B) Two-layer Structured Core (1) Production of Center

[0081] The rubber composition for the center having the formulationshown in Table 2 was mixed, and then vulcanized by press-molding in themold at the vulcanization condition shown in the same Table to obtainspherical center. The diameter, deformation amount, central pointhardness (2 a) and surface hardness (2 b) of the resulting center weremeasured, and the hardness difference (2 b-2 a) was determined bycalculation. The results are shown in the same Table. TABLE 2 Centercomposition (parts by weight) VI VII VIII BR-18 *1 100 100 100 Zincacrylate 33 33 30 Zinc oxide 11.5 11.5 12.5 Diphenyl disulfide *2 0.50.5 0.5 Dicumyl peroxide *3 0.8 0.8 0.8 Vulcanization condition Thefirst stage Temp. (° C.) 170 170 140 Time (min) 15 15 20 The secondstage Temp. (° C.) — — 165 Time (min) — — 8 Diameter of center (mm) 33.639.4 33.6 Center hardness (Shore D) Central point hardness (2a) 33 37 47Surface hardness (2b) 52 58 50 Hardness difference (2b − 2a) 19 21 3Deformation amount 3.50 2.95 3.40 of center (mm)

[0082] The rubber composition for the intermediate layer having theformulation shown in Table 2-2 was mixed, and coated on the centerproduced in the step (1) into a concentric sphere, and then vulcanizedby press-molding in the mold at the vulcanization condition shown in thesame Table to obtain two-layer structured core.

Intermediate Layer Composition (iii)

[0083] The composition for the intermediate layer having the formulationshown in Table 3 was directly injection molded on the center produced inthe step (1) to obtain two-layer structured core. The diameter,deformation amount and surface hardness (2 c) of the resulting two-layerstructured core were measured, and the hardness difference (2 c-2 a) wasdetermined by calculation. The results are shown in Tables 7 and 8.TABLE 3 Intermediate layer composition (parts by weight) i ii iii BR-18*1 80 80 — BR-10 *4 20 20 — Zinc acrylate 37 35 — Zinc oxide 10.5 11.0 —Dicumyl peroxide *3 0.7 0.7 — Hi-milan 1605 *5 — — 60 Hi-milan 1706 *6 —— 40 Vulcanization Temp. (° C.) 170 170 — condition Time (min) 16 16 —

Preparation of Cover Compositions

[0084] The formulation materials for the cover showed in Table 4 weremixed using a kneading type twin-screw extruder to obtain pelletizedcover compositions. The extrusion condition was,

[0085] a screw diameter of 45 mm,

[0086] a screw speed of 200 rpm,

[0087] a screw L/D of 35.

[0088] The formulation materials were heated at 200 to 260° C. at thedie position of the extruder. The hardness were determined, using asample of a stack of the three or more heat and press molded sheetshaving a thickness of about 2 mm from the cover composition, which hadbeen stored at 23° C. for 2 weeks, with a Shore D hardness meteraccording to ASTM D 2240-68. The results are shown as a cover hardnessin Tables 4 to 8. TABLE 4 (parts by weight) Cover composition A B C D EF G Elastollan XNY90A *7 100 50 40 — — — — Elastollan XNY97A *8 — 50 40— — — — Elastollan ET890 *9 — — — 100 — — — Pebax 5533SNOO *10 — — 20 —20 — — Pandex T-1198 *11 — — — — 80 100 — Hi-milan 1605 *5 — — — — — —50 Surlyn 9320 *12 — — — — — — 50 Titanium dioxide 4 4 4 4 4 4 4 Coverhardness 46 47 48 48 52 51 50 (Shore D)

Examples 1 to 12 and Comparative Examples 1 to 12

[0089] The resulting cover compositions were preliminary shaped intosemi-spherical half-shells by injection molding, encapsulating theresulting core with the two half-shells, followed by press-molding inthe mold at 170° C. for 2 minutes to form a cover layer having thethickness shown in Tables 5 and 7 (Examples) and Tables 6 and 8(Comparative Examples). Then, clear paint was coated on the surface ofthe cover layer to obtain a golf ball having a diameter of 42.8 mm and aweight of 45.4 g. With respect to the resulting golf balls, thedeformation amount, coefficient of restitution, flight performance(launch angle, spin amount and flight distance), scuff resistance andyellowing resistance were measured or evaluated. The results are shownin Tables 5 and 7 (Examples) and Tables 6 and 8(Comparative Examples).The test methods are as follows.

Test Method (1) Deformation Amount

[0090] The deformation amount is determined by measuring a deformationamount, when applying from an initial load of 98 N to a final load of1275 N on the center, core or golf ball.

(2) Hardness (i) Center Hardness

[0091] The surface hardness of the center was determined by measuring aShore D hardness at the surface of the resulting center. The centralpoint hardness of the center was determined by cutting the resultingcenter into two equal parts and then measuring a Shore D hardness at itscentral point in section.

(ii) Core Hardness

[0092] The surface hardness of the core was determined by measuring aShore D hardness at the surface of the resulting core obtained byforming the intermediate layer on the center.

(iii) Cover Hardness

[0093] The cover hardness was determined by measuring a Shore Dhardness, using a sample of a stack of the three or more heat and pressmolded sheets having a thickness of about 2 mm from the covercomposition, which had been stored at 23° C. for 2 weeks. The Shore Dhardness was measured using a Shore D hardness meter according to ASTM D2240.

(3) Coefficient of Restitution

[0094] An aluminum cylinder having a weight of 200 g was struck at aspeed of 45 m/sec against a golf ball, and the velocity of the cylinderand the golf ball after the strike were measured. The coefficient ofrestitution of the golf ball was calculated from the velocity and theweight of both the cylinder and the golf ball. The measurement wasconducted 5 times for each golf ball, and the average is shown as thecoefficient of restitution of the golf ball.

(4) Flight Performance

[0095] A No. 1 wood club (W#1, a driver) having metal head was mountedto a swing robot manufactured by True Temper Co. and the resulting golfball was hit at a head speed of 50 m/second, the flight distance wasmeasured. As the flight distance, total that is a distance to the stoppoint of the hit golf ball was measured, and is indicated by an indexwhen that of Example 1 is 100. The measurement was conducted 12 timesfor each golf ball (n=5), and the average is shown as the result of thegolf ball.

(5) Yellowing Resistance

[0096] The resulting golf ball was expose to a sunshine weather metermanufactured by Suga Test Instruments Co., Ltd. for 120 hours. The Labcolor difference (ΔL, Δa and Δb) of the surface of the golf ball betweenbefore and after the exposure was measured by using acolor-difference-colorimeter, which is commercially available fromMinolta Co., Ltd. under the trade name “CR-221”, and was represented byΔE. The ΔE is determined by using the following formula:

ΔE=[(ΔL)²+(Δa)²+(Δb)²]^(½)

[0097] The larger the value of color difference AE is, the less theyellowing resistance is.

(6) Scuff Resistance

[0098] After a pitching wedge (PW) commercially available was mounted toa swing robot manufactured by True Temper Co., two points on the surfaceof each golf ball was hit at a head speed of 36 m/sec one time for eachpoint. The two points were evaluated by checking the surface appearanceby visual observation. The evaluation criteria are as follows.

Evaluation Criteria

[0099] ∘: The surface of the golf ball slightly has a cut, but it is notparticularly noticeable.

[0100] Δ: The surface of the golf ball clearly has a cut, and thesurface becomes fluffy.

[0101] x : The surface of the golf ball is considerably chipped off, andthe surface noticeably becomes fluffy.

Test Results

[0102] TABLE 5 Example No. Test item 1 2 3 4 5 (Core) Composition I I III I Diameter (mm) 41.2 41.2 41.2 41.8 41.8 Deformation amount (mm) 2.952.95 2.95 3.00 3.00 Hardness difference (1b − 1a) 20 20 20 24 24 (Cover)Composition A B C B C Hardness (Shore D) 46 47 48 47 48 Thickness (mm)0.8 0.8 0.8 0.5 0.5 (Golf ball) Deformation amount (mm) 2.90 2.85 2.802.98 2.92 Coefficient of restitution 100 101 102 101 102 Launch angle(degree) 10.5 10.6 10.7 10.9 10.8 Spin amount (rpm) 2800 2750 2700 26802650 Flight distance 100 102 103 103 104 Scuff resistance ∘ ∘ ∘ ∘ ∘Yellowing resistance (ΔE) ∘ ∘ ∘ ∘ ∘

[0103] TABLE 6 Comparative Example No. Test item 1 2 3 4 5 6 (Core)Composition I I I III IV V Diameter (mm) 41.2 41.2 41.2 41.2 41.2 40Deformation amount (mm) 2.95 2.95 2.95 2.70 3.25 2.85 Hardnessdifference 20 20 20 3 10 18 (1b − 1a) (Cover) Composition D E F B B BHardness (Shore D) 48 52 50 47 47 47 Thickness (mm) 0.8 0.8 0.8 0.8 0.81.4 (Golf ball) Deformation amount (mm) 2.90 2.80 2.75 2.65 3.20 2.60Coefficient of 97 98 96 99 96 95 restitution Launch angle (degree) 10.110.0 10.1 10.0 10.3 10.1 Spin amount (rpm) 2850 2630 2900 2950 2800 2850Flight distance 97 98 95 98 97 96 Scuff resistance x x x ∘ ∘ ∘ Yellowingresistance (ΔE) x ∘ ∘ ∘ ∘ ∘

[0104] TABLE 7 Example No. Test item 6 7 8 9 10 11 12 Center compositionVI VI VI VI VII VII VII (Intermediate layer) Composition i i i i iii iiiiii Thickness (mm) 3.8 3.8 3.8 4.1 0.9 0.9 1.2 (Core) Diameter (mm) 41.241.2 41.2 41.8 41.2 41.2 41.8 Deformation 2.80 2.80 2.80 2.75 2.70 2.702.60 amount (mm) Surface hardness (2c) 60 60 60 61 70 70 70 (Shore D)Hardness difference 27 27 27 28 33 33 33 (2c − 2a) (Cover) Composition AB C B B C B Hardness (Shore D) 46 47 48 47 47 48 47 Thickness (mm) 0.80.8 0.8 0.5 0.8 0.8 0.5 (Golf ball) Deformation 2.70 2.65 2.60 2.68 2.602.55 2.55 amount (mm) Coefficient of 100 101 102 103 103 104 104restitution Launch angle (degree) 10.5 10.6 10.7 10.7 10.8 10.9 10.9Spin amount (rpm) 2700 2650 2600 2630 2500 2450 2480 Flight distance 100101 102 103 103 104 104 Scuff resistance ∘ ∘ ∘ ∘ ∘ ∘ ∘ Yellowing ∘ ∘ ∘ ∘∘ ∘ ∘ resistance (ΔE)

[0105] TABLE 8 Comparative Example No. Test item 7 8 9 10 11 12 Centercomposition VI VI VI VI VII VIII (Intermediate layer) Composition i i ii iii ii Thickness (mm) 3.8 3.8 3.8 3.2 0.9 3.8 (Core) Diameter (mm)41.2 41.2 41.2 40.0 40.4 41.2 Deformation 2.80 2.80 2.80 2.90 2.80 3.00amount (mm) Surface hardness (2c) 60 60 60 58 70 57 (Shore D) Hardnessdifference 27 27 27 25 33 10 (2c − 2a) (Cover) Composition D E F B B BHardness (Shore D) 48 51 50 47 47 47 Thickness (mm) 0.8 0.8 0.8 1.4 1.20.8 (Golf ball) Deformation 2.65 2.68 2.60 2.80 2.70 2.85 amount (mm)Coefficient of 97 98 96 97 98 98 restitution Launch angle (degree) 10.110.2 10.1 10.1 10.3 10.1 Spin amount (rpm) 2700 2630 2800 2850 2800 2780Flight distance 97 98 96 97 98 98 Scuff resistance x x x ∘ ∘ ∘ Yellowingx ∘ ∘ ∘ ∘ ∘ resistance (ΔE)

[0106] As is apparent from the results of Tables 5 to 8, the golf ballsof the present invention of Examples 1 to 12, when compared with thegolf balls of Comparative Examples 1 to 12, are superior in reboundcharacteristics, flight performance, yellowing resistance and scuffresistance.

[0107] On the other hand, in the golf balls of Comparative Example 1 and7, since the polyurethane-based thermoplastic elastomer formed by usingaromatic diisocyanate is only used as the base resin for the cover, thescuff resistance is excellent, but the yellowing is very easy to occurand the yellowing resistance is poor.

[0108] In the golf ball of Comparative Example 2, the mixture of thepolyurethane-based thermoplastic elastomer formed by using aliphaticdiisocyanate and the polyamide-based thermoplastic elastomer is used asthe base resin for the cover, the yellowing is difficult to occur andthe yellowing resistance is excellent, but the scuff resistance is poor.

[0109] In the golf balls of Comparative Example 3 and 9, since theionomer resin is only used as the base resin for the cover, thecoefficient of restitution is small and the scuff resistance is poor.

[0110] In the golf balls of Comparative Example 4, 5 and 12, since thehardness difference (b-a) between the surface of the core (b) and thecentral point of the core (or center) (a) is small, high launch angleand low spin amount are not accomplished, which reduces the flightdistance.

[0111] In the golf balls of Comparative Example 6, 10 and 11, since thethickness of the cover is large, the coefficient of restitution issmall, which reduces the flight distance.

[0112] In the golf ball of Comparative Example 8, the polyurethane-basedthermoplastic elastomer formed by using aliphatic diisocyanate is onlyused as the base resin for the cover, the yellowing is difficult tooccur and the yellowing resistance is excellent, but the scuffresistance is poor.

What is claimed is:
 1. A solid golf ball comprising a core and a coverformed on the core, wherein the cover comprises as a base resin (a)polyurethane-based thermoplastic elastomer formed by usingcycloaliphatic diisocyanate, or (b) a mixture of polyurethane-basedthermoplastic elastomer formed by using cycloaliphatic diisocyanate andpolyamide-based thermoplastic elastomer, and has a thickness of not morethan 1.0 mm, and a surface hardness in Shore D hardness of the core ishigher than a central point hardness in Shore D hardness of the core bynot less than
 20. 2. The golf ball according to claim 1, wherein thecore is composed of a center and at least one layer of an intermediatelayer formed on the center, a surface hardness in Shore D hardness ofthe center is higher than a central point hardness in Shore D hardnessof the center by not less than 15, the surface hardness of the core ishigher than the central point hardness of the center by not less than20, and the surface hardness of the core is higher than the surfacehardness of the center.
 3. The golf ball according to claim 1, whereinthe cycloaliphatic diisocyanate is at least one selected from the groupconsisting of 4,4′-dicyclohexylmethane diisocyanate,1,3-bis(isocyanatomethyl)cyclohexane, isophorone diisocyanate, andtrans-1,4-cyclohexane diisocyanate.
 4. The golf ball according to claim1, wherein the cycloaliphatic diisocyanate is 4,4′-dicyclohexylmethanediisocyanate.
 5. The golf ball according to claim 1, wherein the coverhas a Shore D hardness of 35 to 60 and a deformation amount whenapplying from an initial load of 98 N to a final load of 1275 N of 2.50to 3.20 mm.